A magnetic recording device such as a hard disk drive (HDD) used mainly in a computer to record and reproduce information has been applied to various fields such as home video recorder, audio device, and vehicle-mounted navigation system because of its excellent capacitance, cost effectiveness, data accessibility, and data-hold credibility. As HDD technique is being adopted in broader technical fields, higher storage capacity is required, and thus, developments of HDD with higher density have been pursued intensely in recent years.
As a magnetic recording method used in a commercially available HDD, a so-called perpendicular magnetic recording method is the mainstream in recent years. In the perpendicular magnetic recording method, a magnetic recording layer used to record information is made of magnetic crystal grains which have their easily magnetization axes in a direction orthogonal to a substrate of the layer. With such a structure, influence by an inverse magnetic field between recording bits can be reduced even in a densification process and the densification process itself can be performed in a magnetostatically stable condition. The perpendicular magnetic recording medium is generally structured with a substrate, soft magnetic underlayer to concentrate magnetic flux generated from a magnetic head in the recording, perpendicular magnetic recording layer containing a hard magnetic material, non-magnetic seed layer and/or non-magnetic underlayer to place the magnetic crystal grains in (00.1) surface orientation and reduce the orientation dispersion in the perpendicular magnetic recording layer, and protective layer to protect the surface of the perpendicular magnetic recording layer.
A granular-type recording layer has a so-called granular structure in which the magnetic crystal grains are surrounded by grain boundary regions made of non-magnetic materials, and therein, the magnetic crystal grains are two-dimensionally and physically isolated from each other by the non-magnetic grain boundary region and such a structure achieves reduction in magnetic exchange interaction between the magnetic grains. Thus, transition noise in recording and reproducing characteristics can be reduced and a limit bit size can be reduced also. Conversely, since the magnetic exchange interaction between the magnetic grains is reduced in the granular-type recording layer, the dispersion of the inverse magnetic field tends to increase depending on the dispersion of components and sizes of the grains and such a structure causes increase of the transition noise and jitter noise in the recording and reproducing characteristics.
Furthermore, the lower limit value of the recording bit size is set depending largely on the size of the magnetic crystal grains of the granular-type recording layer. Thus, for increasing recording density of the HDD, the grains in the granular-type recording layer are required to be fine-grained. As a method for fining the grains in the granular-type recording layer, it is known to use a technique of using a underlayer having fine-grained crystal grains to fine the grains of the granular-type recording layer layered thereon. Here, for fining the grains of the underlayer, there are several possible methods such as manipulating the non-magnetic seed layer and making the underlayer granular.